SUN worshippers are being warned that wearing sunglasses could increase the risk of skin cancer.

The glasses reduce the amount of ultra-violet light reaching key areas of the eye, tricking the brain into believing less harmful rays have penetrated the body.

The result, doctors believe, is that the body produces less of a natural substance it needs to defend itself against harmful ultraviolet light.

The theory will be published tomorrow in a book called Survival Of The Sickest, which analyses the origins of modern disease.

It was written by disease specialist Dr Sharon Moalem, who has studied the process of tanning and ultra-violet, or UV, light.

Sunglasses make the brain think it's dark and this means you're not starting the natural process of tanning.

Disease specialist Dr Sharon Moalem

He thinks it may help explain the soaring rates of deadly skin cancer, or melonomas, in affluent countries.

His research has alarmed skin experts and led to calls for the development of "clear" sunglasses that protect the eye from UV damage but do not trick the body.

Previously, scientists and dermatologists had urged people to wear sunglasses to shield the retina from damage.

But Dr Moalem said: "Sunglasses make the brain think it's dark and this means you're not starting the natural process of tanning. You're more likely to burn and therefore at more risk of skin cancer."

He acknowledged there were other factors which led to tanning but said: "Light in the eye is an important factor. It helps to short-circuit the body's natural defence mechanism against the sun."

Dr Moalem's theory is borne out by research published in the Journal of Investigative Dermatology suggesting that UV light in the eye activates a chemical known as the melanocyte-stimulating hormone which makes the skin thicken and go brown to protect it from harmful rays.

Professor John Hawk, melanoma expert for the British Skin Foundation, agreed that the production of the bodies' tanning chemicals "will almost certainly be diminished by sunglasses".

He said: "This is a very interesting theory that may well prove to be true and research into it is necessary."

However, he said good quality sunglasses also helped protect the eye against cataracts and other damage.

Professor Dorothy Bennett, an expert on cell biology at St George's University, London, also believed the theory was "entirely possible" but pointed out that it would be difficult to stop people wearing sunglasses.

Dr Sven Krengel, a German expert who has also studied the possible link between wearing sunglasses and increased risk of skin cancer, said: "I like this theory. I think it should be pushed forward for further research."

Dr Krengel published a separate paper in the Journal of Dermatology that suggested wearing sunglasses prevented people seeking shade in bright sunlight. He said this could partly explain why the incidence of skin cancer has been rising.

Last night he said: "Companies should produce sunglasses that are clear but still block damaging UV rays which increase the risk of cataracts and other eye damage."

Exposure to hot sun has been widely said to increase the chances of suffering skin cancer, due to damage posed by ultraviolet rays.

More than a million people in the US were diagnosed with the disease last year, making it the fastest growing cancer risk in America.

Malignant melanomas are one of the deadliest forms of cancer in Britain with 8,100 new cases each year and nearly 2,000 deaths.

Over the past 25 years, the incidence of deadly melanoma has increased more than for any other cancer in the UK.

Male rates have risen from about 2.5 per 100,000 in 1975 to 11.0 per 100,000 in 2003, while female rates have tripled from 3.9 to 12.6 per 100,000 in the same period.

The survival rate can be as high as 99 per cent but only if detected early.

Scientists have warned sunbathers to use sunblock or at least factor 15, wear protective clothes outdoors and avoid direct exposure to the sun when its rays are most intense.

People most vulnerable to skin cancer are those with many moles, with red or fair hair, and those who burn easily in the sun but find it difficult to tan.

The disease affects more women than men, with those aged between 40 and 60 thought to be most at risk, although two0thirds of sun damage is done before people turn 15.